In a MOSFET, doped regions for source and drain are doped oppositely to the surrounding semiconductor material, in which the source/drain regions are embedded and which forms the channel region between the source/drain regions. This can be the semiconductor material with the basic doping of the substrate or a doped well formed in the substrate. The substrate or the well is called the body. The source, channel and drain correspond to the emitter, base and collector of a parasitic bipolar transistor. When this parasitic bipolar transistor becomes conducting, which is called the snapback effect, the blocking voltage and the switching rate of the MOSFET, especially a power MOSFET, are considerably degraded. The parasitic bipolar transistor is switched on by body currents, which cause a voltage drop in the substrate, which leads to a forward voltage of the pn junction between source and body. To eliminate this undesired effect, the source and body are short circuited via an ohmic resistance, for which a body terminal is provided.
This problem is especially serious in the case of symmetric MOSFETs, since their body terminal contacts are located far from the regions in which the body current is produced. Since the gain or amplification factor in the case of a parasitic npn transistor is much greater than in the case of a pnp transistor, mainly N channel MOSFETs are affected by the snapback effect.
Methods for avoiding snapback in the case of high voltage MOSFETs are described in U.S. Pat. No. 5,185,275. A buried p+ layer under the channel is proposed.
U.S. Pat. No. 4,656,493 describes a power MOSFET in which the current amplification of the npn transistor is reduced by providing in its structure a region in which recombination of charge carriers with a relatively low lifetime is provided for the excess majority charge carriers.